Here's the order:
"Public locations include any place open to the public including, without limitation, grocery stores, pharmacies, and other retail stores; public transportation, taxis, livery, and other ridesharing vehicles; public streets and ways; and any location that hosts indoor or outdoor events or performances. Masks or cloth face coverings are also required when in a carpool with non-household members."
I'm pretty sure the Guv has accounted for the toddlers who won't participate. It's like lowering the speed limit from 65 to 55. He's trying to get more compliance. You'll just need to be more vigilant about "being such a cool rebel by not wearing a mask... oooh!" It's not like the police are going to strongly enforce the $300 fine seeing as they're mostly republican (especially in the boonies).
I was worried that he was going to shift back to Phase II, but I'd gladly take the mask order over a rollback. I live in Boston proper, and I see a surprising number of people without masks (including me some days as I just plain forget for my run), as there were too many discrepancies: "We're making this clear for everyone. No more exceptions, and no exemptions for when you can maintain social distance," Baker said. "It's our hope that this simpler, more straightforward rule becomes easier for the public to follow and easier for local officials to enforce this mandatory order this way."
Contact tracing is a absolute joke in the US right now, so we have no idea what the transmission is like, so of course we don't have proof of anything.